Lockout relays of various types are used throughout the electrical power industry. Such relays provide remote trip, manual or remote reset operation. They are used for automatically tripping and locking out circuit breakers and operating other devices when a system failure or fault occurs. Lockout relays are often used in conjunction with differential relays to protect transformers, buses and rotating machinery in electrical power systems.
In service, lockout relays are often maintained in the reset position. In the event of trouble such as short circuits, lightning strikes or overloads, they operate by tripping out. When tripping occurs, it is considered undesirable because system integrity is temporarily lost. However, it is the alternate to more severe consequences such as equipment damage due to excessive voltage or current with resulting power failure. Lockout relays are designed to mechanically latch in the reset position; yet, they must trip out with great speed in response to electrical fault signals.
Conventional lockout relay designs utilize a magnetic tripping solenoid for remote operation of a mechanical latch. For a given operating voltage, these solenoids have fewer turns or coils than do other coil designs. The purpose of this is to minimize the inductance thus reducing the time constant (T=L/R). This allows the current to rise very quickly in the coil when it is energized. The coils have low DC resistance and require very little energy to operate.
These coil characteristics provide lockout relays with high speed operation, with tripping capable of completion in 16 milliseconds or less. Thus system protection can be initiated very quickly.
A disadvantage to using this type of coil is that they are especially susceptible to false operation from low energy electrical transients that sometimes occur with some frequency in electrical power systems.
The transients are capacitive in nature and their occurrence is considered normal. Occurring at random and lasting only a fraction of a second, they can and have caused lockout relays to trip out causing great disruption. This is considered "nuisance tripping". Power failures can occur due to nuisance tripping.
The industry has determined that it is preferred that lockout relays be designed not to operate or trip at one-half or below the normal operating battery voltage. Thus, a lockout relay having a magnetic tripping solenoid used with a 140 VDC battery should not operate at 70 VDC and below.
The half voltage requirement by itself can be satisfied quite simply. However, if the established operating characteristics of conventional relays are to be maintained, as they must for operative purposes, the half voltage requirement is difficult to meet.